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Among seabirds to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Evidence

It is not the first time experts have suggested ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the concept chimed with research that has found humans of non-African ancestry have bits of ancient genetic material in their genome, revealing genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," the lead researcher said.

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how humans smooch.

Defining Kissing

"There have been some previous attempts to describe a intimate act, but it's very much been focused on humans, which implies that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what our intimate contact resembles," explained Brindle.

However, she said some actions that looked like kissing were something rather different – such as the chewing and food sharing, or "mouth contact", seen in fish known as French grunts.

As a result the team developed a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Research Approach

The lead researcher explained they focused on reports of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then integrated this information with details on the evolutionary relationships between extant and ancient species of such animals.

Evolutionary Origins

The team say the findings suggest kissing developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this evolutionary lineage means it is likely they, too, engaged in a kiss, the scientists conclude. But the behavior may not have been limited to their own species.

"The fact that modern people engage intimately, the reality that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have kissed," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle explained kissing could be used in reproductive situations to possibly increase mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range of species might extend its origins back even earlier still.

"Behaviors that we consider as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Elements

Another professor said that intimate contact had a social component as it was not common to all human groups.

"However, as people we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and closeness will have been significant for millions of years," she said. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and including them and our own species collectively – kissed."